The problem of providing an annular or ring patch of fused thermoplastic material bonded to the thread groove surfaces at the interior of a nut has long posed difficulties.
The end desired result is a nut having a solid complete 360.degree. ring of thermoplastic material, such as nylon 11, bonded in the thread grooves to the thread surfaces in such a way as to leave the end convolutions free of deposit to facilitate threading onto a mating article.
In the past, two different solutions to the problem have been proposed. In one the nut is first heated and then dry powdered resin is blown onto the hot threads. This method is disclosed in Duffy U.S. Pat. No. 3,896,760. In the other, dry powdered resin is deposited in the threads, and suitably confined as by a pin, and the entire assembly, including the pin, is subsequently heated while maintaining the pin and nut in a predetermined spacial orientation. Such an operation is disclosed in Newnon U.S. Pat. No. 3,975,897, for example.
Both of these operations, which require the application of dry resin powder to a hot nut, or retaining the powder on the nut while it is heated, require complicated and expensive equipment and are inherently difficult to carry out.
In my prior copending application, Ser. No. 892,505, these objections are overcome and the resin is provided in an efficient manner to the thread surfaces where it may subsequently be fused while the nuts are randomly positioned in bulk. In this prior application, the powdered resin is provided in a fluid slurry which is applied to the threads at the interior of a nut by slinging the slurry radially outwardly from a zone located within the threaded nut opening. In order to obtain quantity production it is necessary to provide for insertion and withdrawal of slinging mechanism relative to the nut opening which in turn requires advancing a series of nuts intermittently.
In accordance with the present invention, a series of nuts are advanced continuously with their thread axes vertical, and the threaded openings are filled with fluid slurry either completely or to a predetermined height, leaving the top few thread convolutions clear of slurry. Thereafter the slurry is permitted to drain downwardly, leaving in the thread grooves up to the level to which the threaded openings were filled a deposit of slurry which assumes an outwardly concave meniscus surface, the precise contour of which is determined in part by the fluidity or viscosity of the slurry. The excess slurry is captured in a drain receptacle for re-use. The nuts are further advanced during which the excess slurry is washed from the exterior of the nuts, and the slurry in the thread grooves will partially dry.
Finally, the slurry is removed from the bottom few thread convolutions, thus leaving an intermediate ring or band of slurry extending around an annular zone within the threaded interior of the nut spaced from both ends thereof.
The liquid carrier of the slurry deposit is substantially eliminated, and the nuts as so far treated contain essentially solid coherent annular deposits of resin particles within the thread grooves of the nuts, in which the deposits are shaped to present outer concave meniscus surfaces, and in which the deposits are essentially formed of particles of thermoplastic resin, such as nylon 11. These nuts are ready for subsequent heating to fuse the resin particles into essentially solid resin deposits fused to the thread surfaces, in which the shape of the initial slurry deposit in the thread grooves is preserved. This is found to be true, even if the pretreated nuts are heated in bulk in which the individual nuts occupy random positions.